基于分层核磁测试新技术的未冻水变化规律研究——以砂土冻融过程为例

Study on the variation law of unfrozen water based on the new technology of layered nuclear magnetic testing:taking the freezing and thawing process of sand as an example

土体冻融过程中的未冻水动态变化与冰-水相变过程密切相关,是冻融过程中非饱和土研究的重要基础。利用在线控温以及分层扫描的核磁共振新技术直观测试冻融过程中非饱和砂土的未冻水含量。结合T_(2)分布曲线(曲线上不同的T;值对应着孔隙水类别特性,曲线下方的面积对应试样水分含量)在冻融过程中的峰值大小和峰面积数据反演土体中含水量的大小与赋存的位置,而曲线的峰形态以及弛豫范围(各峰起始值以及终止值)等信息反演不同类型水分(吸附水与毛细水)以及土体结构的分布。在处理试验结果时,首先依据测试得到的冻结温度划分试样冻结区与未冻区。冻结区与未冻区未冻水含量及其孔隙变化差异明显,究其原因是冰水相变与水分迁移。在土样冻结区域冰水相变占主导地位,水分主要由未冻区向冻结锋面附近的e、f层迁移。首先以中大孔隙中毛细水迁移为主,其次以小孔隙中的吸附水迁移为辅。依据水相变成冰体积增大和孔隙体积占比数据分析可知,冻结区微小孔隙会在冻结过程中连通形成中大孔隙;而在未冻区水分迁移占主导地位。未冻区受固结作用中大孔隙压缩形成为小孔隙。试验过程中冻结锋面附近的e、f层孔隙变化最为剧烈。

The dynamic change of unfrozen water during freezing-thawing process is closely related to the icewater phase transition,which is an important basis for the study of unsaturated soil. In this paper,the unfrozen water content of unsaturated sand during freeze-thaw process is tested by on-line temperature control and layered scanning NMR technology. The peak size and peak area data of the T;distribution curve(different T_(2)values on the curve correspond to the characteristics of pore water category,and the area below the curve corresponds to the moisture content of the sample)in the freezing and thawing process inverse the size and location of water content in soil,and the peak shape and relaxation range of the curve(the initial value and termination value of each peak)inverse the distribution of different types of water(adsorbed water and capillary water)and soil structure.When the test results are processed,the frozen area and unfrozen area of the sample are firstly divided according to the freezing point obtained by the test. There are obvious differences in unfrozen water content and pore changes between frozen and unfrozen areas,which are caused by ice water phase transition and water migration.The ice-water phase transformation is dominant in the freezing area of soil samples,and the water mainly migrates from the unfrozen area to the e and f layers near the freezing front. Firstly,capillary water migration in medium and large pores is dominant,followed by adsorbed water migration in small pores. According to the analysis of the volume increase of water phase into ice and the proportion of pore volume,it can be seen that the tiny pores in the freezing zone will connect to form medium and large pores during the freezing process. Water migration is dominant in the unfrozen area. The large pores in the unfrozen area are compressed into small pores under consolidation. During the test,the pore changes of e and f layers near the freezing front are the most dramatic.